Ocean carbon cycling since the middle Miocene: testing the metabolic hypothesis

呼吸作用-有机物(食物)被分解以提供能量并释放二氧化碳的过程-强烈依赖于温度。温度每升高10度，其发生速度大约快2.5倍。我们是呼吸有机体，但我们没有注意到这一点，因为我们的体温是有调节的，但冷血生物是有调节的，就全球过程而言，最重要的呼吸器--细菌和其他微生物也是如此。这就是为什么我们把食物放在冰箱里，也是为什么热带沼泽比温带沼泽更具生物活性。最近，地球系统的科学家们开始意识到，如果我们要理解一些与气候变化有关的大型全球反馈，就必须考虑到微生物新陈代谢的这种显著的温度依赖性，因此我们应该将其纳入地球系统的计算机模型。帮助调节大气中二氧化碳数量的一个重要过程发生在海洋中，被称为生物泵。藻类在表面的光区进行光合作用，形成食物链的基础。这些有机物中的大部分在表层被吃掉并被呼吸，二氧化碳被返回到大气中，但相当一部分下沉到更深的水中。大多数二氧化碳最终也会被细菌分解，但这里释放的二氧化碳是从表面分离出来的。一些有机物质可以到达海底，在那里它可以并入沉积物，形成未来的碳氢化合物源岩。有机物如雨点般沉入深海和沉积物中，会产生一个补偿性的二氧化碳“泵”，从大气进入海洋。现在想象一下，由于气候变化，我们调高了水柱的温度。这对细菌来说是个好消息，因为它们能更有效地利用下沉的有机物。从表层海洋中移走的碳更少，因此二氧化碳在大气中积累，直到恢复新的平衡。由于二氧化碳是一种重要的温室气体，当它在大气中时会导致全球变暖，从理论上讲，这个过程可能会加剧变暖过程，或者反过来在一个正在降温的星球上起作用。重要的是，我们要了解这种反馈在现实世界中有多么重要，以及它可能对地球系统的其他部分产生什么连锁反应。我们已经设计了一种方法来研究地球的过去，利用海底的化石沉积物。我们计划在大洋彼岸采集过去1500万年的一系列沉积物样本，以调查生物泵的效率。在此期间，地球已经显着冷却，因此我们预测海洋生态系统和生物泵的功能将发生重大变化。我们将研究分布在水柱中的有孔虫(大量出现的微型原生动物)化石外壳的化学成分。通过组合使用地球化学技术，我们可以建立温度分布、pH分布和生物泵的强度。为了探索这些数据，我们将使用一个特别修改的版本的最先进的地球系统模型，该模型将考虑代谢过程的温度依赖性。然后，我们将使用该模型来研究它对一系列全球重要因素的影响，如有机碳埋藏模式和大气二氧化碳，并调查这些因素对未来气候变化的重要性。我们预测，过去1500万年的全球变冷改善了深层浮游生物生态位(即所谓的海洋黄昏地带)的氧气和食物供应，这将刺激深层的进化创新。我们将通过研究浮游生物在时间和空间上的深度模式来检验这一观点，并调查在这种环境中是否有增强的进化。

Respiration - the process by which organic matter (food) is broken down to provide energy, releasing carbon dioxide - is strongly temperature-dependent. For every ten degrees increase in temperature, it occurs about 2 and a half times faster. We are respiring organisms but we don't notice this because our body temperatures are regulated, but cold-blooded creatures do, and so too do the most important respirers of all in terms of global processes - the bacteria and other microbes. This is why we put food in the fridge, and why a tropical swamp is a much more biologically active place than a temperate bog. Recently there has been a dawning realization among Earth System scientists that this marked temperature-dependency of microbial metabolism must be taken into account if we are to understand some of the big global feedbacks involved in climate change, and hence we should incorporate it into Earth System computer models.One important process that helps regulate the amount of CO2 in the atmosphere occurs in the ocean, and is called the 'biological pump'. Algae photosynthesize in the photic zone at the surface, forming the base of the food chain. Most of this organic matter gets eaten up and respired in the surface layer and the CO2 is returned to the atmosphere, but a substantial proportion sinks to deeper water. Most of it does, eventually, also get broken down by bacteria, but here the CO2 released is isolated from the surface. Some of the organic matter can reach the sea floor where it can be incorporated into sediments, forming the hydrocarbon source rocks of the future. The rain of organic matter sinking to the deep sea and sediments produces a compensatory 'pump' of CO2 from the atmosphere to the ocean. Now imagine we turn up the temperature in the water column as a result of climate change. This is good news for the bacteria which use up the sinking organic matter more efficiently. Less carbon gets removed from the surface ocean hence CO2 accumulates in the atmosphere until a new balance is restored. Because CO2 is an important greenhouse gas, contributing to global warming when it is in the atmosphere, this process could theoretically accentuate the warming process, or work the other way round on a cooling planet. It is important that we understand how important this feedback is in the real world, and what knock-on effects it may have in other parts of the Earth System. We have devised a way of studying it in the Earth's past, using fossil sediments from the sea floor. We plan to take a series of sediment samples spanning the last 15 million years across the oceans to investigate the efficiency of the biological pump. The planet has cooled markedly over this period so we predict major changes to the functioning of ocean ecosystems and the biological pump. We will study the chemical composition of fossil shells of foraminifera (microscopic protists that occur in large numbers) that lived distributed through the water column. By using a combination of geochemical techniques we can establish the temperature profile, pH profile, and strength of the biological pump.To explore the data we will use a specially modified version of a state-of-the-art Earth System Model that will take into account temperature-dependency of metabolic processes. We will then use the model to investigate its impact on a range of globally important factors such as patterns of organic carbon burial and atmospheric carbon dioxide, and investigate how important these factors are for future climate change.We predict that global cooling over the last 15 million years has produced improved oxygenation and food supply in deep planktonic niches (the so-called 'twilight zone' of the ocean) and that this would have spurred evolutionary innovation at depth. We will test this idea by studying plankton abundance patterns at depth in time and space and investigating whether there has been enhanced evolution in this environment.